There have been many recent efforts to develop structures for focusing electromagnetic waves into sub-wavelength spots in the near-field [A. Salandrino, and Engheta, “Far-field sub diffraction optical microscopy using metamaterial crystals: Theory and simulations”, Physical Review B, August 2006, 74, pp. 075103; A. Grbic and G. V. Eleftheriades, “Overcoming the diffraction limit with a planar left-handed transmission-line lens,” Phys. Rev. Lett., vol. 92, no. 11, pp. 117403, Mar. 19, 2004; P. Alitalo, S. Maslovski and S. Tretyakov, “Experimental verification of the key properties of a three-dimensional isotropic transmission-line superlens,” JAP, vol. 99, p. 124910, 2006]. In particular, near-field sub-wavelength focusing has been demonstrated by artificially synthesized metamaterial lenses, which come close to operating according to the ‘perfect lens’ concept developed by J. B. Pendry [J. B. Pendry, “‘Negative refraction makes a perfect lens”, Phys. Rev. Lett., April 2000, 85, (18) pp. 3966-3969]. Such near-field focusing structures hold promise for improving microscopy, lithography, near-field sensing, imaging, and other applications. However, all these efforts are hindered by material losses which severely limit the achieved sub-wavelength resolution levels.
Some researchers propose near-field focusing using an assumed aperture current distribution obtained by manipulating the corresponding plane-wave spectrum [R. Merlin, “Radiationless electromagnetic interference: evanescent-field lenses and perfect focusing”, Science, July 2007] and a physical realization has recently been reported [A. Grbic, L, Jiang, R. Merlin, “Near-Field Plates: Subdiffraction Focusing with Patterned Surfaces,” Science, vol. 320, no. 5874, pp. 511-513, Apr. 25, 2008]. While these structures appear to hold promise for a variety of applications including microwave imaging, near-field sensing, lithography, and microscopy, they remain of limited practical use because they cannot be scaled to any arbitrary frequency and the demonstrated focusing is confined in a parallel-plate environment (and not free space). Moreover the demonstrated focusing is limited to one dimension (azimuth only).
Near-field imaging systems capable of producing sub-wavelength resolution have also attracted much interest in the photonics community. Their ability to focus the evanescent components of an illuminated object breaks the conventional barrier of the “diffraction limit”, and leads to the formation of concentrated sub-wavelength light spots on the order of nanometers. Such structures can find applications that range from bio-imaging and sensing to photolithography.